Ice is one of those strange things where the crystal structure of ice is much larger than the liquid structure. So, ice expands when it freezes. Most things shrink. Think of molten metal of any kind, when you pour the liquid into a mold, it will get smaller when it solidifies (freezes) so you can get it out of the mold. If you throw the piece back into the liquid metal pot it will sink to the bottom.

If you pour water into a mold and freeze it, the stuff will expand and break the mold.

Some people on here were talking about air bubbles. This simply is not true, although there are air bubbles in most ice. But ice from the bottom of a glacier has had all of the air squeezed out and it still floats because of the ice structure.